Offshore_technology

1. Introduction
2. Size and configuration
3. Support structure
4. Standards
5. Project Experience
6. Operation and maintenance
7. Electrical
8. general REFERENCES

CA-OWEE 1.4 Offshore Technology | Standards

4 Standards

4.1 General

The issue of building permits for offshore wind turbines will depend on a large number of different agencies and institutions. This is not only due to the different technical fields involved, but also due to the impact from the marine environment (navigation, national parks, pipelines, cables, defence areas, etc.). Many European countries have appointed one authority to co-ordinate the necessary involvement of the relevant organisations. In most countries this appointment is also different depending on the distance to the shore, i. e. local, inside 12 miles or outside.

In Europe the technical design of wind turbines shall be based on the relevant European Directives. Of special importance for wind turbines is the Machinery and the Construction Product Directives. However, the Low Voltage and Electromagnetic Compatibility Directives also need to be satisfied. All of these Directives are general purpose documents which ask for harmonised standards and requirements.

A European set of building codes are the Eurocodes 1, 2, 3 which are published as ENV 1991, 1992, 1993. The Eurocodes are based on the method of analysing limit states according to ISO 2394 and do require the use of partial safety factors. Eurocode 1 defines loads, Eurocode 2 contains requirements for concrete structures and Eurocode 3 those for steel structures.

In addition to the existing IEC-standards, the European Directives, Eurocodes and a number of national codes for wind turbines, Germanischer Lloyd’s Regulation for the Certification of Offshore Wind Energy Conversion Systems [1] and the Danish Recommendation for Technical Approval of Offshore Wind Turbines [25] give guidance on the special design requirements for offshore wind turbines. Further national and international codes and regulations for offshore structures may be applicable.

The design of offshore wind turbine foundations can be based on the long term experience gained in projects undertaken by the oil and gas industry. However, it has to be pointed out that for existing offshore structures, wind is generally not one of the dimensioning load components. The structural design of the offshore wind turbine has to take into account both wind loads and the structural response of the foundation which may result from waves, currents or ice.

Extended remote control is one of the design modifications for offshore wind turbines. Others are corrosion protection against marine atmosphere, boat or helicopter landing facilities and lifting gear for components.

Design rules for offshore wind turbines have been derived from codes for wind turbines and those for offshore structures. Although there is considerable experience for both of those structures their combination has revealed new load cases which need to be considered in the design, construction and operation of offshore wind farms.

4.2 GL Offshore Standard

Germanischer Lloyd’s (GL) Regulations for the Certification of Offshore Wind Energy Conversion Systems (GL-OW) [1], issued 1995, are a result of the Joule 1 Offshore study [5] by merging the GL Regulations for the Certification of Wind Energy Conversion Systems (GL-W) and the Rules for Offshore-Installations (GLO). The structure and main components of these Regulations are described in [6].

In the meantime since the first issue of the regulation, new knowledge has been gathered on offshore wind and wave conditions and some pilot wind farms have been constructed. There is a strong requirement to bring the GL-OW Regulations in line with new developments.

Review of the Regulations is underway consisting of following points:

1. Resolve insufficiencies and errors found in planning and certification procedures:
Several offshore wind farms are in the planning or design stage.. These include wind farms in Denmark, Germany and the Netherlands where Germanischer Lloyd WindEnergie GmbH (GL-Wind) is actively incorporated as a certification body.

2. Incorporate results from applications in pilot farms: GL-Wind is participating in the EU research project ‘Offshore Wind Turbines at Exposed Sites’ (OWTES), being undertaken by AMEC Border Wind, Delft University of Technology, Germanischer Lloyd WindEnergie, PowerGen Renewables Developments and Vestas Wind Systems under the leadership of Garrad Hassan and Partners [8].

The aim of this project is to improve the design methods for wind turbines located at exposed offshore sites in order to facilitate the gradual, cost-effective exploitation of the offshore wind energy resource available in the EU. This aim will be met through the achievement of a number of project objectives. These include to;

· establish a database of environmental and structural load measurements.

· evaluate the database of environmental and structural measurements in order to derive a thorough understanding of the aerodynamic and hydrodynamic loads and their influence on the dynamic response of the offshore wind turbine and its support structure.

· use the database of measurements to enable validation and enhancement of state-of-the-art-methods for computer modeling and design analysis of offshore wind turbines.

· undertake parametric analyses for investigation of the complex relationships between fatigue and extreme loading, the design characteristics of an offshore wind turbine and its support structure, and the site wind, wave, current and sea bed conditions.

· investigate the robustness of design calculations for offshore wind turbines with respect to variations in the environmental conditions, wind turbine and support structure design concepts and methods of analysis.

· provide a critical appraisal of present design procedures and certification rules for offshore wind turbines and to recommend changes where appropriate.

· catalogue the key design requirements for offshore wind turbines for sites where the environmental conditions are severe.

The database of measurements recorded at Blyth Harbour is evaluated in order to establish a complete characterisation of the environmental conditions at the site. The characterisation will identify the correlation of wind, waves and currents. In addition, the spectral characteristics of the wind turbulence and the wave heights will be established and compared with the standard models recommended by the certification regulations for offshore wind turbines.

The measurements of environmental data and structural response will be used to examine the extent to which the assumptions underlying the current GL certification regulations for offshore wind turbines are valid for the Blyth Harbour site.

A thorough review of the current GL certification regulations for offshore wind turbines will be undertaken. Based on a critical evaluation of the project results, the validity of the assumptions and guidelines offered by the GL regulations will be examined and, where appropriate, recommendations for revision will be made.

3. Update according to scientific / technological progress.

A number of research projects have provided valuable information on offshore specific issues. Specific subjects have been investigated separately e.g. wind resources, extreme wind and to some extent wave conditions, turbulence characteristics, joint-appearance (probability) of wind, waves, ice and current and on operation and maintenance. Some of the results are now available [9], [10], [11], [12], [13], [14], [15] and the effort is to include these in future regulations updates.

4. Harmonization with IEC.

Considerable work has been performed by the IEC TC 88 committee, resulting in the second edition of the IEC 61400-1 in 1999 [7]. According to this standard, offshore wind turbines have to be treated as land based wind turbines of class “S”, considering marine environment. As most offshore turbines are “marinised” versions of land based turbines developed in accordance with IEC 61400-1, a harmonisation with the IEC code is of advantage. This task is scheduled for 2001-2002 and will be performed as a review of the regulations for land based wind turbines [2]. In Parallel GL-Wind is participating in the relevant national and international working groups of DIBt, CENELEC, IEC TC88 for offshore (WG03) and land based wind turbines (WG01) which will have influence on the regulation harmonisation.

4.3 Danish Recommendation for Technical Approval of Offshore Wind Turbines(Rekommandation for Teknisk Godkendelse af Vindmøller på Havet)

The Danish Energy Agency has issued recommendations for the approval of offshore wind farms in Denmark. Generally the standard DS472 applies, with significant changes in some parameters. A short description of the recommendation is given here:

Part 1: Introduction, applicable standards. Wind turbines to be erected offshore Denmark have to fulfill the Technical Criteria for Type Approval and Certification of Wind Turbines in Denmark, The Danish Standard DS472 and other norms and regulations stated in the Technical criteria. For the analysis of wave loading, DS449 (Piled offshore structures) and for ice loading API 2N [26] have to be applied. Further Danish national construction norms (DS409 – DS415) to be considered are named.

Part 2: Climatic parameters and safety in relation to DS472. The changes of parameters relative to DS472 are described. Annual mean and extreme wind speed as a function from distance to shore, air density and safety factors for the loads to be used for offshore windturbines are stated. Additionally a method to be used for the calculation of wind farm influence on wind speed turbulence intensity is given.

Part3: Loads and load cases. The calculation methods and the nature of the dynamic model are described together with the loads acting on the structure. Depending on the system sensitivity some guidance on analysis methods and extent is given. Apart from the definition of the characteristic values (98% of the annual extreme value) and the coefficient of variation to be used together with safety factors, a list of load cases, based on DS472 and extended for offshore climate is stated. Recommendations on the combination of wind, wave, ice and current loading and the extraction of design loads from them are included.

Part 4: Foundations. Reference is made to DS415 (Foundation) and DS 449 (Piled offshore structures). The determination of the geotechnical category, the required pre-appraisals like measurements or laboratory experiments are considered together with inspection requirements.

Part 5: Materials and corrosion. This section refers to the protection systems and durability of the support structure up to the nacelle. Corrosion protection is considered. Regulations to be applied for concrete and steel structures are listed.

Part 6: Additional conditions such as occupational safety, lightening protection, marking, noise emission and environmental impact assessment are stated.

4.4 IEC Offshore Wind Turbine Standards

4.4.1 Review

According to the existing IEC 61400-1 standard, offshore wind turbines have to be treated as land based wind turbines of class “S”. This is not a satisfactory solution and the Technical Committee 88 of the IEC set up a working group (WG03) to develop IEC 61400-3 specially dedicated to offshore wind turbines.

4.4.2 Objective of WG03

The objective of WG03 is to develop a standard for the engineering and technical requirements which should be considered during design in order to ensure the safety of systems and components of offshore wind turbines, inclusive of their support structures. This will be documented in IEC 61400-3.

IEC 61400-3 will cover only those issues relevant to offshore wind turbines, fully consistent with IEC 61400-1 and not duplicating the requirements defined in IEC 61400-1.

4.4.3 Contents

The contents of the document will be limited (at the beginning) to offshore wind turbines with support structures which are fixed to the seabed (not floating systems). It is proposed that a wind turbine be considered “offshore” if the support structure is subject to hydrodynamic loading. The main issues to be considered are: external conditions, design load cases, calculation methods, structural design, and assembly, installation erection, commissioning and maintenance.

The time schedule agreed in WG03 is shown in the following table:

Status of IEC 61400-3	Proposed Target Date
Availability of first WD (working draft)	December 2001
Circulation of first CD (committee draft)	June 2002
Submission of first CDV (committee draft for voting)	December 2002
Submission of FDIS (final draft international standard)	December 2003
Availability of IS (international standard)	June 2004

Table 4.4.3.1 Time Schedule of WG03

4.5 Offshore Environment

Apart from general rules and regulations on offshore wind turbine design, site specific environmental conditions are of interest. The influence of wind, wave, ice and soil conditions is covered by the standards for offshore, offshore wind turbine and land based wind turbine designs, together with procedures for site assessment. The certification procedure according to the site conditions is given in [1] and [16] and described in [6].

In addition to the standards normally applied for land based machinery, electrical machinery and buildings, the following may be of interest.

· Electrical conditions may have significant impact on wind turbine design, especially in conjunction with weak grid conditions. National standards or grid operator requirements will regulate electrical parameters to be fulfilled by the wind farm and the electrical installation up to the connected point on land. Additionally the grid loss probability and duration may (directly) influence load definitions in the standards.

· Operation and Maintenance and related labour safety issues are also covered by national regulations. They will have influence in access and rescue equipment and boarding platforms.

· The marine atmosphere must be considered for corrosion, as well as guidance relating to the materials to be used and electrical protection.

· Ship navigation will not directly influence turbine structural design except the collision case. National laws and international agreements determine the equipment to be installed (light marking, active and passive radar reflectors etc). The ship collision probability and load has to be considered.

· Installation, lifting and commissioning are generally covered by offshore regulation although national regulations may apply.

· Marine pollution, MARPOL, e.g. access visits must be minimised to reduce use of fossil fuels and disturbance on sea fauna.

· Dismantling. In most countries a full dismantling of offshore constructions is required by national law. In Germany by the mining law (§55(2) Nr3 Bberg).

· Air traffic markings in accordance with international and national regulations have to be installed.

· The noise problem cannot be neglected even offshore. Many large scale turbines can produce noise similar to sound levels generated from motorways.

· Site specific approach wind+wave+ice+soil conditions.

· Procedures on site assessment and certification according to GL and IEC.

· Electrical conditions – power supply power company, National O&M National Work safety influence on safety systems, accessibility, platforms etc.

· Shipping, navigation, air traffic national and international regulations and their influence on design e.g. collision, site spec. depth etc.

· Lightning protection requirements.

4.6 Offshore Industry Standards

Standards that will apply or assist in installation and erection procedures and in the design of special structures not included in wind energy related codes. These are listed in the following:

Offshore regulations

1. American Petroleum Institute, Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Working Stress Design, API Recommended Practice 2A-WSD, 21^st Edition 2000.

2. American Petroleum Institute (API), Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Load and Resistance Factor Design, 1993, (suppl. 1997), RP 2A-LRFD

3. American Petroleum Institute, Recommended Practice for Planning, Designing and Constructing Structures and Pipelines for Arctic conditions, API Recommended Practice 2N, 2nd Edition 1995.

4. Norwegian Technology Center (NTC), NORSOK Standard N-001, Structural Design, Rev. 3, Aug. 2000.

5. Department of Energy, (now Health and Safety Executive) 1990: Offshore installations: guidance on design, construction and certification (fourth edition) HMSO 1990 ISBN 011 4129614, replaced.

6. Det Norske Veritas, Rules for classification of fixed offshore installations 1998.

7. Germanischer Lloyd, Rules for Classification and Construction, III Offshore Technology, 2 Offshore Installations, Edition 1999

8. ISO 13819-1, Petroleum and natural gas industries -- Offshore structures -- Part 1: General requirements, 1995-12, 1st edition. To be replaced , ISO TC 67. (ISO 19900)

9. ISO 13819-2 Petroleum and Natural Gas Industries – Offshore Structures – Part 2: Fixed steel structures, 1995.

10. ISO 19903 (Draft), Offshore Structures – Fixed concrete structures.

Offshore Mobile Platforms

1. Det Norske Veritas, Rules for classification of mobile offshore installations.

2. Germanischer Lloyd, Rules for Classification and Construction, III Offshore Technology, 2 Offshore Installations, Guidelines for the Construction/Certification of Floating Production, Storage and Off-Loading Units, Edition 1999.

3. IMO, MODU-Code, Code for the construction and equipment of mobile offshore drilling units, 1989.

4. ISO 19904 (Draft), Offshore Structures – Floating systems.

Electrical Equipment

1. American Petroleum Institute, Recommended Practice for design and installation of electrical systems for Offshore.

2. IEC 60092-xxx (2000-02) Electrical installations in ships

3. IEC 60533 (1999-11) Electrical and electronic installations in ships - Electromagnetic compatibility

4. IEC 60654-2 (1979-01) Operating conditions for industrial-process measurement and control equipment. Part 2: Power

5. IEC 60654-4 (1987-07) Operating conditions for industrial-process measurement and control equipment. Part 4: Corrosive and erosive influences

6. IEC 61363-1 (1998-02) Electrical installations of ships and mobile and fixed offshore units - Part 1: Procedures for calculating short-circuit currents in three-phase a.c

7. IEC 61892-3 (1999-02) Mobile and fixed offshore units - Electrical installations - Part 3: Equipment

8. IEC 61892-6 (1999-02) Mobile and fixed offshore units - Electrical installations - Part 6: Installation

Materials and Corrosion

1. DIN EN 12495, Cathodic protection for fixed steel offshore structures, 2000.

2. DIN EN 10225, Weldable structural steels for fixed steel offshore structures, 1994.

3. Det Norske Veritas, R.P. B401, Cathodic Protection Design, 1993

4. Germanischer Lloyd, Rules and Regulations, II Materials and Welding, Part 1, Metallic Materials, Edition 1998.

5. Germanischer Lloyd, Rules and Regulations, II Materials and Welding, Part 1, Non-metallic Materials, Edition 2000.

Special Topics

1. IMO, Safety of Life at Sea Convention (SOLAS)

2. Marine pollution , MARPOL

3. International Association of Sea-Mark Administrators (AISM/IALA) Recommendations for the marking of offshore structures, Nov. 1984 /suppl. 1987).

Helicopter Platforms

1. Cap 437, Offshore Helicopter Landing Areas.

2. American Petroleum Institute, Recommended Practice for Planning, Designing and Constructing Heliports for Fixed Offshore Platforms, API Recommended Practice 2L, 4^th Edition 1996.

Offshore Cranes

1. American Petroleum Institute, Specification for Offshore Cranes, API Spec 2C, 5^th Edition 1995.

2. DIN EN 13852, Cranes – Offshore Cranes – Part 1: General purpose offshore cranes, 2000

4.7 EU-Project Recommendations for Design of Offshore Wind Turbines (RECOFF)

The objective of this project is to prepare guidelines and recommendations for design of offshore wind turbines. The main objective of these guidelines and recommendations is that they should serve as a basis for development of European and national standards and certification rules for offshore wind turbines. The recommendations will be addressed directly to the two standardisation bodies: the International Electrotechnical Commission (IEC) and the European CENELEC.

The existing offshore standards, mainly written for offshore oil and gas exploitation, are not suitable to cover the offshore wind energy technology. Particular review of health and safely issues for offshore work on OWECS must ne a priority. A combination of these offshore standards and the existing onshore wind energy standards is in process but technology gaps exist. In the project, readily available information will be utilized to the extent possible, and where a need is identified, research and development will be performed. The project is structured in accordance with the typical components of a standard. The main tasks are reflected in the project work packages:

1) External conditions: identification and description of wind, waves, ice etc.,

2) Computational tools: generation of loads from external conditions,

3) Design load cases: identification of a suitable number of representative load cases,

4) Probabilistic methods: new models for decision-making on load cases,

5) Structural integrity: specification of e.g. partial safety coefficients,

6) Operation and maintenance: labor safety and standard method for data collection.

7) Project management and communication: management, preparation and execution of seminars for external parties such as manufacturers.

The proposed work (3 years duration) will aim to bring together available information and expert knowledge from the wind power ( Riso (coordinator), CRES, ECN, GH and GL) and offshore engineering industries. The overall methodology of the project is summarized in Figure 4.7.1.

Figure 4.7.1 : Overview of the Methodology used in the Project ^[1].

4.8 References

1. Germanischer Lloyd, Rules and Regulations, IV Non Marine Technology, Part 2 Regulations for the Certification of Offshore Wind Energy Conversion Systems, Edition 1995.

2. Germanischer Lloyd, Rules and Regulations, IV Non Marine Technology, Part 1 Regulations for the Certification of Wind Energy Conversion Systems, Edition 1999.

3. Germanischer Lloyd, Rules for Classification and Construction, III Offshore Technology, 2 Offshore Installations, Edition 1999.

4. Germanischer Lloyd, Rules for Classification and Construction, III Offshore Technology, 2 Offshore Installations, Guidelines for the Construction/Certification of Floating Production, Storage and Off-Loading Units, Edition 1999.

5. Matthies et al, „Study of Offshore Wind Energy in the EC, Final Report Joule I (JOUR 0072), Verlag Natürliche Energie 1995.

6. C. Nath, “Experiences in Offshore Certification”, Proceedings of the EUWEC Göteborg 1996.

7. IEC 61400-1, ed. 2, Wind Turbine Generator Systems, Part1 – Safety Requirements, Feb. 1999.

8. T.R. Camp, D.C. Quarton, “Design Methods for Offshore Wind Turbines at Exposed Sites”, JOR-CT98-0284.

9 Bitner-Gregersen, E.M., Hagen, O., "Aspects of Joint Distribution for Metocean Phenomena at the Norwegian Continental Shelf", Proceedings of ETCE/OMAE2000, ASME 2000.

10. Myrhaug D. Slaattelid O.H., "Wind Stress over Waves: effects of sea roughness and atmospheric stability", Proceedings of ETCE/OMAE2000, ASME 20000.

11. Matthies et al, "Offshore Windkraftanlagen: Kombination der Lasten von Wind und Wellen", TU Braunschweig 2000.

12. Timco G.W., et al, "The NRC Ice Load Catalogue", Proceedings of 15th Int. Conference on Port and Ocean Engineering under Arctic Conditions, POAC'99, Vol 1, pp 444-453, Helsinki Finlad.

13. Crespo, A., R. Gomex-Elvira, S. Frandsen and S Larsen (1999) Modelisation of large wind farm, considering the modification of the atmospheric boundary layer, 1999 European Wind Energy Conference and Exhibition, Nice France, March.

14. Frandsen, S. and K. Thomsen (1997) Change in Fatigue and Extreme Loading when Moving Wind Farms Offshore; OWEMES '97, Sardinia, Italy, April.

15. Frandsen, S. (Editor), L. Chacon, A. Crespo, P. Enevoldsen, R. Gomex-Elvira, J.H Ýjstrup, F. Manuel, K. Thomsen and P S Ý rensen (1996) Measurement on and Modelling of Offshore Wind Farms, Ris Ý-R-903(EN) report.

16. IEC 61400-22, Wind Turbine Certification.

17. American Petroleum Institute (API), Fixed offshore platforms, Working Stress Design, 1993

18. American Petroleum Institute (API), Fixed offshore platforms, Load Resistance Factor Design, 1989.

19. Draft ISO 13819-2 Petroleum and Natural Gas Industries - Offshore Structures - Part 2: Fixed steel structures.

20. IMO, MODU-Code, Code for the construction and equipment of mobile offshore drilling units, 1989.

21. Cap 437, Offshore Helicopter Landing Areas.

22. Det Norske Veritas, Rules for classification of fixed offshore installations.

23. IMO, Safety of Life at Sea Convention (SOLAS).

24. Health & Safety Executive: Offshore installations: guidance on design, construction and certification (fourth edition) HMSO 1990 ISBN 011 4129614.

25. Danish Recommendation for Technical Approval of Offshore Wind Turbines (Rekommandation for Teknisk Godkendelse af Vindmøller på Havet), Danish Energy Agency 2001.

26. API Recommended practice 2N, “Recommended practice for planning, designing and constructing structures and pipelines for arctic conditions”, 1995.

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